The present invention relates to a ferroelectric capacitive element using, as a capacitor dielectric film, a bismuth-based layered perovskite type ferroelectric, that is, a dielectric metal oxide, thin film (hereinafter simply referred to as the ferroelectric thin film), and more particularly, it relates to a structure of the ferroelectric thin film and a method for forming the ferroelectric thin film.
Recently, there is a trend toward using an oxide film of a bismuth-based, and specifically, an SBT-based or a BST-based layered perovskite type as a ferroelectric thin film working as a capacitor dielectric film. Such an oxide film is applied to a semiconductor electronic device such as a RAM employing hysteresis of polarization inversion by utilizing the ferroelectric characteristic of the ferroelectric thin film. A ferroelectric capacitive element using such a ferroelectric thin film as a capacitor dielectric film is affected by annealing performed in fabrication of the ferroelectric capacitive element. Specifically, out diffusion is caused in the composition of a ferroelectric thin film through the annealing. Accordingly, it has become a significant problem to prevent degradation of the ferroelectric polarization characteristic by preventing the out diffusion of the composition of the ferroelectric thin film caused in the annealing.
In a conventional technique employed for overcoming this problem, a conductive ceramic film is used as at least one of an upper electrode and a lower electrode provided to a ferroelectric thin film and a platinum thin film is provided between the ferroelectric thin film and the conductive ceramic film. Thus, composition shift of the ferroelectric thin film caused through diffusion of a ferroelectric composite included in the ferroelectric thin film is prevented, so as to suppress the degradation of the ferroelectric characteristic (see, for example, Patent document 1: Japanese Laid-Open Patent Publication No. 10-27888 (p. 3, paragraph 15).
Now, a semiconductor device including a ferroelectric capacitive element according to the conventional technique disclosed in Patent document 1 will be described with reference to FIG. 14.
FIG. 14 is a cross-sectional view for showing the structure of the semiconductor device including the ferroelectric capacitive element of the conventional technique.
As shown in FIG. 14, on a substrate 100 made of a semiconductor substrate of monosilicon or the like or an insulating substrate of quartz or the like, a lower ferroelectric capacitor dielectric film electrode 101 made of a multilayered film including a conductive ceramic film 101a and a platinum thin film 101b formed in this order in the upward direction is formed. A ferroelectric capacitor dielectric film 102 is formed on the lower ferroelectric capacitor dielectric film electrode 101. An upper ferroelectric capacitor dielectric film electrode 103 made of a multilayered film including a platinum thin film 103a and a conductive ceramic film 103b formed in this order in the upward direction is formed on the ferroelectric capacitor dielectric film 102.
In this case, a semiconductor thin film of polysilicon or amorphous silicon, an interconnect layer, or an insulating film of SiO2, Si3N4 or the like may be formed on the substrate 100. Also, the conductive ceramic film 101a or 103b is made from a rutile type oxide such as RuO2, IrOx or OsO2 or a perovskite type composite oxide such as SrRuO3, SrIrO3 or ReO3. Furthermore, the ferroelectric capacitor dielectric film 102 is made from a lead-based perovskite type composite oxide such as PbTiO3, Pb(Zr2Ti1-x)O3 or PbyLa1-y(ZrxTi1-x)O3, a barium-based perovskite type composite oxide such as BaxSr1-xTiO3, or a bismuth-based layered composite oxide such as SrBi2Ta2O9 or Bi4Ti3O12.
In this manner, the platinum thin film 101b is disposed on the interface between the conductive ceramic film 101a and the ferroelectric capacitor dielectric film 102, and the platinum thin film 103a is disposed on the interface between the conductive ceramic film 103b and the ferroelectric capacitor dielectric film 102. Thus, the platinum thin films 101b and 103a prevent diffusion of elements included in the ferroelectric capacitor dielectric film 102, and therefore, the composition shift of the ferroelectric capacitor dielectric film 102 through the diffusion of a ferroelectric composite is prevented, so as to suppress the degradation of the ferroelectric characteristic.
According to this publication, the structure of the ferroelectric capacitive element shown in FIG. 14 is disclosed as a basic structure, the ferroelectric capacitor dielectric film electrode including the conductive ceramic film and the platinum thin film may be formed as merely one of the upper and lower electrodes, and the structure of the ferroelectric capacitor dielectric film electrode is not limited to the two-layered structure but may be a multilayered structure including a metal thin film of Ru, Ir or the like or a metal compound thin film of TiN, WSi or the like.